The overall goal of this project is to understand the relationship between membrane architecture and function in the photosynthetic membrane of the blue-green alga, Synechococcus cedrorum. The problem is approached using a combination of genetic, molecular and physical techniques. A broad range of temperature-sensitive, photosynthetic mutants have been isolated using the antibiotic metronidazole as an enrichment agent. These mutants include some that are defective in Photosystem (PS) I or II activity, and some that have enhanced PS activity. The complete characterization of these mutants including fluorescence kinetics and spectra, protein composition and arrangement and electron transport composition is now being pursued. The precise arrangement of proteins in the photosynthetic membrane is being studied by specific labeling and analyzed via one- and two-dimensional acrylamide gel electrophoresis. External membrane proteins are labeled with I125 by lactoperoxidase, while proteins embedded in the lipid bilayer are labeled with I125-Iodonaphthylazide. The labeling studies will also be performed on isolated photosystem particles and on membranes from mutants. The use of quinones as electron acceptors and donors in whole cells is being investigated, as is the interaction of metronidazole with the electron transport chain.